## Fast Mental Maths tricks

Everything from basic math up through high school!

Moderators: Bob Hazen, Theodore, elliemaejune

koconnor
User
Posts: 2
Joined: Sat Feb 09, 2008 2:31 pm
Contact:

### Fast Mental Maths tricks

Hi all

Here are some maths "tricks" for you to fall in love with!

How do we multiply by eleven easily?

First we will try this by writing the answer backwards.

So, when always multiplying a large number by 11, the last number will always be the last number of the sum we are multiplying.

Let me explain.

176 is the number we are multiplying.

So the last number in out answer will be 6. If the sum was 143 the last number in our answer would be 3â€¦ get it?

So now what?

Easy, just add the next number to the last number, 7 + 6 = 13

So the next last number will be 3, carry the 1.

Add the 7 and 1 together, giving 8, then add the carried 1, 9 then write that number as our next part of the answer, 936

Then, the last number will be the first number in our sum, in this case 1.

Try 278Last number is 8

7 + 8 = 15

Second last number is 5, carry the 1

7 + 2 = 9 add the carried 1, 10

Third last number is 0, carry the 1

Add the carried 1 to the first number in the sum, 2 giving:

2 + 1 = 3

Alternatively, you could always just add a zero to the number you are multiplying and then add the original number to it againâ€¦

156 X 11

try it this way:

Add a zero to the end

1560

1560 + 156 = 1716

Whatever method works best, stick to it!

This was taken from the attached pdf document... Enjoy.

http://www.memorymentor.com/Memorymento ... s_tips.pdf
Fall in love with maths again!

http://www.vedic-maths-ebook.com

Mathmom
User
Posts: 87
Joined: Tue Nov 06, 2007 9:02 am
Hi,
Thanks for sharing.
I'm glad to see the 11's trick, again. It's the one I have trouble doing in my head. Do you actually do it in your head? Or do you put it down on paper until you get better at it? What about with the other ones you have in the pdf?
My 9 yr old son loves doing math in his head. Sometimes he uses tricks he learned. He uses whatever one is easiest, like you say to do.
It doesn't look like we've learned all of these on your pdf.
Thank you for sharing.
I recommmend parents to learn these tricks along with their children. They are very useful.
I didn't see the trick for multiplying by 5's. Do you know that one?
Just wondering.

Mathmom
User
Posts: 87
Joined: Tue Nov 06, 2007 9:02 am
I like the 11's multiplication trick. It just takes some time to remember it, if you don't practice it.
The multiplying by 5's trick is cool. Say you want to do 34 times 5.
5 = 10/2, so do 34 x 10 = 340; 340/2=170; therefore, 5 x 34 = 170.
See, you don't need a calculator

seekingmyLord
User
Posts: 231
Joined: Wed Jul 04, 2007 4:44 am
Location: Standing in the radiance of His glory.
We have not yet started multiplication, but we begin simple multiplication next month if all goes well with my lesson plans. --Amazing how they just keep on learning! It was just last spring that she really struggled with the concept of subtraction!

Right now my daughter really into adding and subtracting 9's, as in 4+9=13 because 3 is on less than 4 and then we simply add 10 and 17-9=8 because 8 is one more than 7 and the 10 is gone.

She also is into using doubles, as in 7+7=14 so 7+8=15.

It is just so cool to watch her thinking it out!

Mathmom
User
Posts: 87
Joined: Tue Nov 06, 2007 9:02 am
Hi Seeking,
Are you doing this adding/subtracting on paper? With pictures? Manipulatives, or are you doing mental math?
Just wondering because we do almost all verbal math problems that we make up. We do have a lot of books about math topics, though.
I like seeing how much fun they have playing around with numbers. It's sad to hear so many others(not you) saying they and their kids don't like math.
Let's keep making it fun.

seekingmyLord
User
Posts: 231
Joined: Wed Jul 04, 2007 4:44 am
Location: Standing in the radiance of His glory.
Mathmom wrote:Hi Seeking,
Are you doing this adding/subtracting on paper? With pictures? Manipulatives, or are you doing mental math?
Just wondering because we do almost all verbal math problems that we make up. We do have a lot of books about math topics, though.
I like seeing how much fun they have playing around with numbers. It's sad to hear so many others(not you) saying they and their kids don't like math.
Let's keep making it fun.

Oh, Mathmom, I would not know where to start to explain what we have done and do.

My daughter understood numbers as we had done some worksheets and manipulatives for pre-school, but I think using Professor B really helped her to recognize the number of items without actually counting--it requires them to hold up different fingers to make a number. I found that she was great on visualizing of how to break down and add up to 5+5, but weaker on adding numbers over 6 because you can only hold up five fingers on one hand. However, I then simply switched over to manipulatives.

Then we moved on to using Singapore (traditional workbook with pictures) as a spine combined with Miquon (creative thinking and manipulatives). However, she also does QuarterMile Math, which is all mental work. She can add and subtract with verbal math problems. I also do drills from time to time using Calculadder. I also have Number Neighbors, which will be very helpful with regrouping, which we will be using probably in a week or two.

I kind of attack math from all angles and see which method she naturally prefers for each concept, but I continually rotate them, because she tends to change her reasoning methods as her skills develop.

I do all this probably because I really enjoy math and I really want her to enjoy it also. My daughter memorizes very easily, which is ideal with most every other subject, and although I could have thrown math tables at her and she would be impressive with giving back an answer, I was concerned that she would not have the understanding of the concepts. So, I purposely stayed away from using a memorization method in the beginning. Because math was not something she could memorize, she really did not like math at all a year ago. However, I just kept telling her she was a math whiz while working through different methods and now she gets it better with each new concept and is looking forward to multiplication. She doesn't exactly enjoy math as much I would like--or so she says, although I think it is growing on her--but she does enjoy getting all the answers right, so that is her motivation with math at this time.

I also am surprised at how many homeschooling parents claim to dislike math. I think math is a fun subject to teach, because it can be done in a such variety of ways.

scatteredpearl
User
Posts: 5
Joined: Thu Feb 14, 2008 11:26 am
Location: Michigan
Contact:

### Multiplying by 9's

Does everyone know the 9's trick?

4x9=

Hold up you ten fingers, and put down the corresponding finger.

l l l l l l l l l l

In this case 4.

4
l l l i l l l l l l

4
l l l i l l l l l l
123 1 123456
3 6
36

4x9=36

Works with all single digits but can be used for larger multuplying, just remember to carry.

kolds
User
Posts: 4
Joined: Thu Mar 20, 2008 11:15 am
Location: Texas
Contact:

### This isn't as much a trick as much as just fun

Rhyme the answers to addion or multipication facts. For instance 2+2 take of your shoe. Equals 4 at the door.

My son loves this. He is in 2nd grade.